Rain is just airborne bacteria鈥檚 way of getting home. That鈥檚 one startling conclusion from an analysis showing that rain-making bacteria are widely distributed in the atmosphere.
Particles of soot and other tiny pieces of inorganic debris are important 鈥渟eeds鈥� of precipitation. It鈥檚 why particles of silver iodide and dry ice are sometimes used to encourage rainfall. The idea that bacterial cells could have the same function is not new, but what has not been appreciated until now is the extent to which biological particles are involved in rainfall.
鈥淭hey are everywhere in the atmosphere,鈥� says , of Louisiana State University, Baton Rouge. 鈥淚t鈥檚 hard to believe they couldn鈥檛 have some impact on precipitation.鈥�
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The particles that Christner is referring to come from bacteria with proteins on the surface of the cell that facilitate ice formation.
Encouraging ice
The bacterium Pseudomonas syringae, for example, has a protein which binds water molecules together in a pattern that mimics the lattice pattern of molecules that forms in ice.
By doing so it facilitates ice formation at warmer temperatures than would otherwise be the case. The ice then either melts on the way down to form rain or falls as snow or hail.
Christner and colleagues collected snow soon after it had fallen from 19 locations, in France, the US and Antarctica, and found evidence of ice-forming proteins and DNA-containing cells at all sites. .
The results mean that rain-making bacteria are present in the atmosphere, and probably in clouds too.
Drought protection
Christner鈥檚 colleague Pierre Amato has sampled directly from clouds, and found that they contain viable bacteria, although whether they are rain-makers is not yet known. 鈥淭here鈥檚 no reason [rain-making bacteria] couldn鈥檛 be in clouds too,鈥� says Christner.
This suggests the intriguing possibility that the surface protein has evolved as part of the bacterial life cycle. Many bacteria get swept up into the atmosphere, and although there are nutrients and water present in clouds, bacteria finding themselves up there might not want to stay.
鈥淚f a bacterium gets aerosolised into the atmosphere and has this protein on its surface,鈥� says Christner, 鈥渋t could facilitate its own precipitation.鈥�
Pseudomonas syringae grows on the leaves of plants and is well known as a plant pathogen. But Christner says it should be possible to plant crops that use other rain-making bacteria to seed their own clouds, and make their own rain.
鈥淚n places that suffer drought you could plant crops that harbour bacteria to increase precipitation,鈥� he says.
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